SizeMyGenerator

Well Pump Starting Watts Guide

Understand running watts, starting watts, locked-rotor amps, and how to size a generator that can reliably start and run your well pump.

What Are Well Pump Starting Watts?

Well pumps require two different wattage values: the running watts needed once the motor is up to speed, and the starting watts required to overcome initial inertia. Starting watts are typically the larger number and are the main factor in generator sizing.

Starting watts are closely tied to the motor’s locked-rotor amps (LRA). You can often find LRA on the pump’s nameplate or control box. If not, the pump manufacturer may publish typical values for the horsepower and motor type.

Running Watts vs. Starting Watts

Residential well pumps commonly operate at 240 volts. To estimate watts, use the basic formula: 

Watts = Volts × Amps

If a pump draws 8 amps running at 240 volts: 

Running watts ≈ 8 A × 240 V = 1,920 W

If the same pump has an LRA of 24 amps: 

Starting watts ≈ 24 A × 240 V = 5,760 W

Typical Starting Watts by Pump Horsepower

These ranges align with common residential pump nameplates:

Pump HP Running Watts (Approx.) Starting Watts (Approx.)
1/2 HP 700–1,100 W 2,000–4,000 W
3/4 HP 1,000–1,600 W 3,000–5,000 W
1 HP 1,500–2,000 W 4,000–6,000 W

These values vary by motor design, manufacturer specifications, and wiring distance. Always verify with the pump nameplate when possible.

Submersible vs. Jet Pump Starting Load

Submersible pumps typically have higher starting surge than jet pumps of similar horsepower due to motor type and pumping head. However, both types can require 2–3 times running watts at startup.

For example, a typical 3/4 HP jet pump might require:

Why Starting Surge Varies

Starting surge depends on:

If your pump is aging or the well is far from your home’s electrical panel, lean toward the upper end of typical generator sizing ranges.

How to Reduce Starting Surge

Some options may help reduce starting demand:

How to Size a Generator for a Well Pump

The safest sizing method is to calculate running and starting watts, then choose a generator where expected peak load is about 70–80% of total rated output. This mirrors best practices for residential optional standby systems under NEC Article 702.

Step 1: Determine Running Amps

Check the nameplate for full-load amps (FLA). Convert to watts:

Running W = FLA × 240 V

Step 2: Determine Starting (Surge) Amps

If you see “LRA,” multiply by 240 volts:

Starting W = LRA × 240 V

Step 3: Add Your Other Running Loads

Include only the circuits you intend to power during an outage. For a full analysis, try the generator sizing calculator.

Step 4: Add Headroom

Add roughly 20–30% headroom to account for temperature, aging equipment, and measurement uncertainty.

For a step-by-step example, see the Home Generator Sizing Guide.

Common Mistakes to Avoid

For safe installation practices, see your local electrical code authority and the National Fire Protection Association, publisher of NEC (NFPA 70).

For portable generator safety, including CO risk, see the U.S. Consumer Product Safety Commission.

Example Calculations

Example for a 3/4 HP submersible pump:

Running: 6.5 A × 240 V ≈ 1,560 W
Starting: 22 A × 240 V ≈ 5,280 W
Other loads during outage: 1,200 W (fridge, lights)
Peak load = 5,280 + 1,200 = 6,480 W
Recommended generator = ~9–10 kW for 20–30% headroom

For your exact numbers, enter your pump details into the generator sizing calculator.

Final Thoughts

Well pumps are one of the reasons generator sizing is not purely based on square footage. Their starting surge can be several times running wattage, so a generator must be chosen with real-world peak load in mind.

For broader context, you may also want to read:

← Back to Home